Why is .NET exception not caught by try/catch block?

Asked 2008-08-30T14:57:13.050

I'm working on a project using the ANTLR parser library for C#. I've built a grammar to parse some text and it works well. However, when the parser comes across an illegal or unexpected token, it throws one of many exceptions. The problem is that in some cases (not all) that my try/catch block won't catch it and instead stops execution as an unhandled exception.

The issue for me is that I can't replicate this issue anywhere else but in my full code. The call stack shows that the exception definitely occurs within my try/catch(Exception) block. The only thing I can think of is that there are a few ANTLR assembly calls that occur between my code and the code throwing the exception and this library does not have debugging enabled, so I can't step through it. I wonder if non-debuggable assemblies inhibit exception bubbling? The call stack looks like this; external assembly calls are in Antlr.Runtime:

    Expl.Itinerary.dll!TimeDefLexer.mTokens() Line 1213 C#
    Antlr3.Runtime.dll!Antlr.Runtime.Lexer.NextToken() + 0xfc bytes 
    Antlr3.Runtime.dll!Antlr.Runtime.CommonTokenStream.FillBuffer() + 0x22c bytes   
    Antlr3.Runtime.dll!Antlr.Runtime.CommonTokenStream.LT(int k = 1) + 0x68 bytes
    Expl.Itinerary.dll!TimeDefParser.prog() Line 109 + 0x17 bytes   C#
    Expl.Itinerary.dll!Expl.Itinerary.TDLParser.Parse(string Text = "", Expl.Itinerary.IItinerary Itinerary = {Expl.Itinerary.MemoryItinerary}) Line 17 + 0xa bytes C#

The code snippet from the bottom-most call in Parse() looks like:

     try {
        // Execution stopped at parser.prog()
        TimeDefParser.prog_return prog_ret = parser.prog();
        return prog_ret == null ? null : prog_ret.value;
     }
     catch (Exception ex) {
        throw new ParserException(ex.Message, ex);
     }

To me, a catch (Exception) clause should've captured any exception whatsoever. Is there any reason wh

c# .net exception antlr

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3 Answers

Is it possible that the exception is being thrown in another thread? Obviously your calling code is single threaded, but maybe the library you are consuming is doing some multithreaded operations under the covers.

answered 2008-08-30T15:10:36.170

Personally I'm not convinced by the threading theory at all.

The one time I've seen this before, I was working with a library which also defined Exception and the usings I had meant that the actual Catch was referring to a different "Exception" type (if it had been fully qualified it was Company.Lib.Exception but it wasnt because of the using) so when it came to catching a normal exception that was being thrown (some kind of argument exception if I remember correctly) it just wouldn't catch it because the type didn't match.

So in summary, is there another Exception type in a different namespace that is in a using in that class?

EDIT: A quick way to check this is make sure in your catch clause you fully qualify the Exception type as "System.Exception" and give it a whirl!

EDIT2: OK I've tried the code and concede defeat for now. I'll have to have another look at it in the morning if no one has come up with a solution.

answered 2008-09-02T16:00:45.823

I downloaded the sample VS2008 project, and am a bit stumped here too. I was able to get past the exceptions however, although probably not in a way that will work will great for you. But here's what I found:

This mailing list post had a discussion of what looks to be the same issue you are experiencing.

From there, I added a couple dummy classes in the main program.cs file:

class MyNoViableAltException : Exception
{
    public MyNoViableAltException()
    {
    }
    public MyNoViableAltException(string grammarDecisionDescription, int decisionNumber, int stateNumber, Antlr.Runtime.IIntStream input)
    {
    }
}
class MyEarlyExitException : Exception
{
    public MyEarlyExitException()
    {
    }

    public MyEarlyExitException(int decisionNumber, Antlr.Runtime.IIntStream input)
    {
    }
}

and then added the using lines into TimeDefParser.cs and TimeDefLexer.cs:

using NoViableAltException = MyNoViableAltException;
using EarlyExitException = NoViableAltException;

With that the exceptions would bubble into the fake exception classes and could be handled there, but there was still an exception being thrown in the mTokens method in TimeDefLexer.cs. Wrapping that in a try catch in that class caught the exception:

            try
            {
                alt4 = dfa4.Predict(input);
            }
            catch
            {
            }

I really don't get why wrapping it in the internal method rather than where it is being called from handle the error if threading isn't in play, but anyways hopefully that will point someone smarter than me here in the right direction.

answered 2008-09-02T22:27:54.363

Why is .NET exception not caught by try/catch block?

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